Term taken from https://en.wikipedia.org/wiki/Pomodoro_Technique meaning a short period where you're working hard. The basic idea of the technique is that you dedicate 25 minutes to strictly working, and then explicitly give yourself a 5 minute break to do whatever. The idea is if you carve out frequent break time you'll be more efficient and focused than if you're trying to sustain 8 hours of solid focus.
It's never worked for me. It takes me 15 minutes to get into a flow. Then the timer rings in 10 minutes and breaks it. An hour of solid work before a break might make more sense for programming.
I think there's something to be said for breaking perseveration. I frequently find myself repeating the same wrong solution to a problem, then breaking for the day and solving it in 5 minutes when I start the next day. I don't think a timer is necessarily helpful. Rubber Ducking, pair programming or the venerated technique of asking for help work pretty well too.
We've found these to be extremely effective. We don't do them all the time but if we feel like we're losing productivity we'll do a few days of them and we're back on track.